Subsurface sea hides below ice of Saturn moon

FROZEN Enceladus, a 500-kilometer-wide moon of Saturn, is blanketed in a thick sheet of ice. Salty water erupts through a network of fissures (blue) in the southern hemisphere. New measurements of the moon’s gravity reveal a subsurface ocean 30 to 40 kilometers beneath the south pole.

Scuba divers take note: an underground ocean awaits on a moon of Saturn. Astronomers have, for the first time, measured the depth and extent of a subsurface sea on the ice-covered moon Enceladus. The findings shore up the notion that an underground reservoir feeds the moon’s ice geysers and raise questions about Enceladus’ habitability.

For a long time, astronomers thought the 500-kilometer-wide Enceladus was an unchanging, dead world. But the Cassini spacecraft, which arrived in 2004, found an active moon. Geysers shoot particles of salty water ice through fissures that dot the southern hemisphere. The fissures expand and contract in rhythm with the tides of Saturn (SN Online: 7/31/13). Heat wells up from the moon’s interior through the cracks. Putting these characteristics together, many astronomers suspected that a sea of liquid water lurked beneath the ice. But those suspicions were based on observations of the moon’s surface.

To peek inside Enceladus, Luciano Iess, an aerospace engineer at Sapienza University of Rome, and colleagues looked at Doppler shifts in Cassini’s Earth-bound radio signal during three flybys of the moon. The Doppler shifts, tiny changes in the frequency of the radio waves, track the spacecraft’s speed. Whenever Cassini passed over a part of the moon with slightly more mass, the increased gravity accelerated the probe. Iess and his colleagues used the changes in Cassini’s speed to map Enceladus’ interior structure.

Iess’ team concluded that a 10-kilometer-deep ocean must sit under 30 to 40 kilometers of ice and on top of the moon’s rocky core. Extending from the south pole to mid-southern latitudes, the sea has a volume of water similar to Lake Superior’s.

Based on previous observations of Enceladus, Iess says he’s not surprised that the gravity data suggest an underground sea. “I would have been more surprised if we didn’t find anything,” he says. The results appear in the April 4 Science.

“This is a major piece of the puzzle,” says Candice Hansen, a planetary scientist at the Planetary Science Institute in Tucson. Until now, researchers had only a trickle of clues hinting at the possibility of a subsurface ocean. “It’s one thing to think that’s what’s happening,” says Hansen, “but it’s another thing to have data that says, ‘Yeah, we’re right.’”

One thing that is not yet clear is how the sea feeds the ice geysers tens of kilometers above it. “It seems unlikely that you’d have a lake directly connected to the surface,” says planetary scientist Joseph Spitale, also of the Planetary Science Institute. “There must be an intermediate plumbing system.”

Spitale adds that the new gravity data appear to rule out one of the leading explanations for how the moon’s interior generates heat: a slight rocking motion induced by Saturn’s gravity. The new data provide no evidence for any rocking.

Regardless of how the interior stays warm, Iess says some interesting chemistry may go on where the sea’s water meets rock. Any discussion of heat rising up from the moon’s interior mixed with a salty ocean inevitably leads to musings on the possibility for life. Though no evidence suggests that aliens swim the seas of Enceladus, the presence of a warm, briny environment raises intriguing possibilities.

For the immediate future, though, those ideas will remain theoretical. Cassini has only three more flybys of the moon planned. “We’re essentially done with Enceladus,” says Iess. “Unfortunately, it will be a long time before we return to a very interesting moon.”

Editor's Note: This story was updated on April 10, 2014, to correct the number of Cassini's planned flybys of Enceladus.